1. Field of the Invention
The present invention relates to a current source circuit, and more particularly to a current source circuit and a method of outputting output currents, which is used for current-driving an apparatus such as an organic EL apparatus.
2. Description of the Related Art
An organic EL display apparatus is of a spontaneously luminous type, and has a faster response speed. Also, the organic EL display apparatus has features of a thin shape, a light weight, and a wide viewing angle. For these reasons, the organic EL display apparatus is suitable for a video display and has high quality. As shown in FIG. 1, in a passive matrix (PM)-type organic EL display apparatus, each pixel is composed of only an organic EL device and wiring lines, and in an activist matrix (AM)-type organic EL display apparatus, each pixel is composed of an organic EL device and a pixel circuit which supplies the organic EL device with a current.
In the organic EL display apparatus, horizontal scanning is repeated in accordance with a signal from a horizontal scanning control circuit to select an organic EL device or a pixel circuit on the horizontal line. During a period corresponding to the horizontal line, a suitable voltage or current is supplied to each organic EL device or each pixel circuit on the horizontal line from a drive circuit for the organic EL display apparatus via data lines. A current that flows through the organic EL device is determined based on the supplied voltage and current, so that the brightness of the organic EL device is controlled for a display. The brightness and the supplied current to the organic EL device are in a linear relation and the brightness and the applied voltage are in a non-linear relation. Also, in the organic EL apparatus in the present situation, the brightness characteristics are degraded with the elapse of time and the brightness decreases. The time change of the brightness in case of the current supply is low compared with a case of the voltage application. Therefore, it is possible to achieve a higher quality display in a current drive method to the organic EL apparatus.
In the activist matrix-type organic EL display apparatus, it is necessary to suppress a deviation of currents supplied from drive transistors to a pixel circuit of an organic EL apparatus for prevention the decrease of the display quality, even if there is a deviation in the current characteristic of the drive transistors. For example, in the voltage application method to a pixel circuit shown in FIG. 2, the currents supplied to the organic EL devices are deviated depending on a deviation of drive transistors in the characteristic. At this time, the brightnesses of the organic EL devices are deviated so that unevenness in color appears on the display. On the other hand, the pixel circuit shown in FIG. 3 is composed of a mirror circuit of a mirror transistor and a drive transistor. Therefore, if there is no deviation between the mirror transistor and the drive transistor, a difference in the current supplied from a drive circuit to the organic EL device can be suppressed.
In the above-mentioned situation, as the drive circuit for driving the organic EL device or the pixel circuit, a drive circuit is proposed which has a digital-analog conversion function to output an analog current in accordance with a digital display data. As such drive circuits, there are a first type drive circuit shown in FIG. 4 which requires a single reference current for a single output current and a second type drive circuit shown in FIG. 5 which requires a plurality of reference currents for the single output current.
The first type drive circuit shown in FIG. 4 is composed of a mirror circuit of a mirror transistor supplied with a single reference current and a plurality of output transistors whose channel widths are different to have a suitable current drive ability ratio. A switch is connected with a drain of each output transistor and is switched based on in accordance with the digital display data. In this case, a summation of the currents from the output transistors which are turned on is outputted, as shown by arrows in a lower portion of FIG. 4.
Also, the second type drive circuit shown in FIG. 5, a mirror circuit is provided for each of the reference currents and has a mirror transistor and an output transistor. Thus, the output currents with a proper current ratio are outputted from the output transistors. At this time, a switch connected with a drain of each output transistor is switched in accordance with the digital display data. Thus, a summation of the currents from the output transistors which are turned on is outputted, as shown by arrows in a lower portion of FIG. 5.
In the above second type drive circuit, the transistors for the current mirror circuit can be arranged closer, because the current mirror circuit is provided for every reference current. Therefore, a deviation of the transistors in the characteristic due to the manufacturing processes can be suppressed low and the precision of the output currents can be improved. On the other hand, in the first type drive circuit, because the mirror transistor is single and the plurality of output transistors exist, current mirror is composed by equal to or more than two plurality of transistors, a deviation of the transistors in the characteristic due to the manufacturing processes is large, compared with the second type drive circuit. Therefore, the precision of the output currents is low, compared with the second type drive circuit.
FIG. 6 shows a third type drive circuit, in which a plurality of reference currents with a suitable current ratio are required like the second type drive circuit. This conventional example is disclosed in EuroDisplay 2002 Proceeding (pp. 279-281). In the third type drive circuit, not a current mirror circuit but a current copier circuit is adopted. The current copier circuit has two operation states, i.e., a current setting operation and a current outputting operation. In the current setting operation, a reference current is supplied to an output transistor in the state that the gate and drain of the output transistor are short-circuited. In this way, the gate voltage of output transistor is set and held to a voltage corresponding to the reference current. In the current outputting operation, a path between the gate and drain of the output transistor is opened and an output current with a same value of as the reference current can be outputted based on the set gate voltage. In this way, the current copier circuit can theoretically output the current with the same value as the reference current regardless of the characteristic of the transistor, because a single transistor is used. The third type drive circuit can suppress an output current deviation caused by a deviation of the transistors in characteristics, compared with the second type drive circuit.
The display quality of the organic EL display apparatus depends on the current which the drive circuit supplies to an organic EL device or a pixel circuit. Therefore, it is possible to improve the display quality of the organic EL display apparatus by adopting the above second or third drive circuit. However, in the above second and third drive circuits, the current source circuit is required to supply a plurality of reference currents with a suitable current ratio to the drive circuit.
In addition to the above description, a conventional current source circuit is disclosed in Japanese Laid Open Patent Application (JP-P2000-293245A). This conventional current source circuit can generate a plurality of reference currents to the above drive circuit. As shown in FIG. 7, this conventional current source circuit is composed of a V-I conversion circuit including an operation amplifier, a current setting resistance Rc, and a transistor Tr1, and a current mirror circuit including transistors Tr2 to Tr5. The V-I conversion circuit operates to supply a current having a value obtained by dividing a voltage applied to the non-inversion input of the operation amplifier by the resistance Rc, to a wiring line of the transistors Tr1 and Tr2 and the resistance Rc. In the current mirror circuit, because the transistors Tr3 to Tr5 have the same voltages between the gate and the source, the transistors Tr3 to Tr5 flow currents determined based on their current abilities and the current flowing through the mirror transistor Tr2. Therefore, if the channel length is same in the transistors Tr3 to Tr5 and a ratio of the channel widths of the transistors Tr3 to Tr5 is 1:2:4, the transistors Tr3 to Tr5 can supply the currents with 1 time, 2 times and 4 times of the current flowing through the transistor Tr2.
Also, Japanese Laid Open Patent Application (JP-P2000-148089A) discloses a technique similar to the Japanese Laid Open Patent Application (JP-P2000-293245A).
Also, Japanese Laid Open Patent Application (JP-P2003-066904A) discloses a technique in which a current is time-divided into N currents by transistors POUT1 to POUTN arranged redundantly to suppress a deviation of outputs from current sources.
Also, Japanese Laid Open Patent Application (JP-P2003-066906A) discloses a technique similar to the Japanese Laid Open Patent Application (JP-P2003-066904A).
In addition, a drive circuit for a current driven type display panel is disclosed in Japanese Laid Open Patent Application (JP-P2003-122307A). In this conventional example, the drive circuit is formed of a polysilicon TFT (thin film transistor) integrated circuit, and uses a current mirror circuit in which a value of a current which flows through the thin film transistor on a reference side is copied to the thin film transistor on a mirror side. The threshold of the thin film transistor on the reference side and the threshold of the thin film transistor on the mirror side are detected for every predetermined period. A change in the threshold of the thin film transistor on the reference side and the threshold of the thin film transistor on the mirror side is corrected based on the detection result. Thus, a change in the current value of the current mirror circuit is corrected.
In the above conventional example, the output currents of the current source circuit are determined based on a ratio of the current abilities between the mirror transistors Tr2 and the output transistors Tr3 to Tr5. However, even if the ratio of the current abilities is set by changing the channel widths, there is a case that the ratio of the current abilities is not a value as designed. In this case, the precision of the output currents decreases. For example, when the transistors are formed as a LTPS TFT (Low Temperature Polysilicon Thin Film Transistor) or an a-Si TFT (Amorphous Silicon TFT), the precision decreases largely, because these transistors have a large deviation of the current characteristic.